Impact of increasing methyl branches in aromatic hydrocarbons on diesel engine combustion and emissions

Lignocellulosic materials have been identified as potential carbon–neutral sources of sustainable power production. Catalytic conversion of lignocellulosic biomass results in liquid fuels with a variety of aromatic molecules. This paper investigates the combustion characteristics and exhaust emissions of a series of alkylbenzenes, of varying number of methyl branches on the monocyclic aromatic ring, when combusted in a direct injection, single cylinder, compression-ignition engine. In addition, benzaldehyde (an aldehyde (-CHO) branch on the monocyclic ring) was also tested. All the molecules were blended with heptane in different proportions, up to 60% wt/wt. The tests were conducted at a constant engine speed of 1200 rpm, a fixed engine load 4 bar IMEP, and at two injection modes: constant start of fuel injection at 10 CAD BTDC, and varying fuel injection timing to maintain constant start of fuel combustion at TDC. The results showed that the ignition delay period increased with increasing number of methyl branches on the ring, due to the rapid consumption of OH radicals by the alkylbenzenes for oxidation to stable benzyl radicals. Peak heat release rates, and concurrently NOx emissions, initially increased with increasing methyl branches, but subsequently both decreased as the bulk of heat release occurred further into the expansion stroke with significant thermal energy losses. With the exception of toluene, the number of particles in the engine exhaust increased as the number of methyl branches on the aromatic ring increased, attributable to the formation of thermally stable benzyl radicals

Combustion and exhaust emission characteristics, and in-cylinder gas composition, of hydrogen enriched biogas mixtures in a diesel engine

This paper presents a study undertaken on a naturally aspirated, direct injection diesel engine investigating the combustion and emission characteristics of CH4-CO2 and CH4-CO2-H2 mixtures. These aspirated gas mixtures were pilot-ignited by diesel fuel, while the engine load was varied between 0 and 7 bar IMEP by only adjusting the flow rate of the aspirated mixtures. The in-cylinder gas composition was also investigated when combusting CH4-CO2 and CH4-CO2-H2 mixtures at different engine loads, with in-cylinder samples collected using two different sampling arrangements. The results showed a longer ignition delay period and lower peak heat release rates when the proportion of CO2 was increased in the aspirated mixture. Exhaust CO2 emissions were observed to be higher for 60CH4:40CO2 mixture, but lower for the 80CH4:20CO2 mixture as compared to diesel fuel only combustion at all engine loads. Both exhaust and in-cylinder NOx levels were observed to decrease when the proportion of CO2 was increased; NOx levels increased when the proportion of H2 was increased in the aspirated mixture. In-cylinder NOx levels were observed to be higher in the region between the sprays as compared to within the spray core, attributable to higher gas temperatures reached, post ignition, in that region

The effect of varying EGR and intake air boost on hydrogen-diesel co-combustion in CI engines

This paper presents a H2-diesel fuel co-combustion study undertaken on a supercharged, direct injection, diesel engine investigating the combustion characteristics and emissions production at a range of engine loads (IMEP), EGR levels and intake air boosting conditions. The utilisation of EGR and intake air boost with H2-diesel fuel co-combustion allows simultaneous NOx and particulate emissions reduction at conditions closer to on-road driving conditions.The results showed that while H2 can be favourable in reducing CO2 and particulate emissions, it causes an increase in NOx emissions when the intake energy contribution from H2 is increased. A reduction in the number of fine and ultrafine particles (diameter 0.05-0.2 μm) was observed when H2 was added to the engine, especially at the low and intermediate intake air boost levels. At high EGR levels (equivalent to 2% intake O2 concentration reduction) significant reductions in exhaust particulate mass of up to 75% were observed at 15% energy from H2. An attempt was made to identify the optimum H2 operating window at the different engine loads, intake air boost and EGR levels

Influence of combusting methane-hydrogen mixtures on compression–ignition engine exhaust emissions and in-cylinder gas composition

The paper presents an experimental investigation of combusting methane-hydrogen mixtures, pilot-ignited by diesel fuel, on a naturally aspirated, direct injection compression ignition engine. The tests were performed with two diesel fuel flow rates for pilot-ignition, and the engine was supplied with different quantities of methane-hydrogen mixtures (in various proportions) to vary the engine load between 0 and 7 bar IMEP. In addition, engine in-cylinder gas samples were collected with two geometric sampling arrangements and at various instants during the engine cycle, to measure species concentrations within the engine cylinder. The results showed lower exhaust CO2 and particulate emissions at all engine loads when combusting methane-hydrogen mixtures as compared to diesel fuel only. CO and unburned THC emissions were higher for methane-hydrogen mixtures at all engine loads when compared with diesel fuel only. NOx emissions increased with increasing proportion of hydrogen in the aspirated mixture at all engine loads. In-cylinder NOx levels were observed to be higher in the region between the fuel sprays as compared to within the spray core, attributable to higher temperatures reached in between the sprays post ignition

Dynamic Response of Acoustically Forced Turbulent Premixed Biogas Flames

Increasing demand for energy and the need for diversification of fuels used in gas turbine power generation is continuing to drive forward the development of fuel-flexible combustion systems, with particular focus on biomass derived sustainable fuels. The technical challenges arising from burning sustainable fuels are largely associated with the change in the chemical, thermal and transport properties of these fuels due to the variation of the constituents and their impact on the performance of the combustor including emissions, static and dynamic stabilities. There is a lack of detailed understanding on the effect of fuel composition on the flame sensitivity to acoustic and flow perturbations. This paper describes an experimental study investigating the acoustic flame response of simulated biogas (methane/carbon dioxide/air mixtures) turbulent premixed flames. The effect of variation in carbondioxide, CO2, content on the flame response was quantified. Special emphasis was placed on understanding the dependence of this flame response on the amplitude of the acoustic forcing. The flame was subjected to strong velocity perturbations using loud speakers. It was observed that the addition of CO2 had considerable influence on the magnitude of heat release response. The magnitude and the phase of flame describing function indicated that the mechanism of saturation in these flames for all conditions tested were the same. The difference in magnitude could been attributed to dilution effect and hence further investigation were carried out with N2 and Ar to clarify the role of CO2. The results indicate that the thermal capacity of the diluent gases could be playing a significant role in nonlinear flame dynamics

Co-combustion of diesel and gaseous fuels with exhaust emissions analysis and in-cylinder gas sampling

The development of novel strategies for improved efficiency and ‘cleaner’ emissions from internal combustion engines requires new insights into the processes of energy release and in-cylinder species formation from future sustainable fuels. This work presents an experimental investigation carried out on a compression-ignition engine to study the co-combustion of diesel fuel with various gaseous fuels, including hydrogen, methane-hydrogen mixtures, biogas and biogas-hydrogen mixtures. In addition, a novel in-cylinder gas sampling and analysis system, developed during the project, was used to extract samples from the engine cylinder during combustion of these gaseous fuels at various stages of the engine cycle, with two sampling location arrangements relative to the diesel fuel spray. Furthermore, the co-combustion of hydrogen and diesel fuel was investigated with the engine intake air boosted and simulated EGR applied. It was found that exhaust NOx emissions were minimal at low engine loads when in-cylinder gaseous fuel-air stoichiometry was quite lean, but increased rapidly when the combined temperatures from gaseous fuel and diesel fuel co-combustion exceeded the threshold of NOx formation temperatures. The eventual level of exhaust particulate mass emissions was observed to be dependent on two competing factors; the aspirated gaseous fuel reduced the intake oxygen concentration resulting in increased soot formation rate, while thermal soot oxidation rates increased due to higher gaseous fuel combustion temperatures. During the early stages of combustion (10 CAD ATDC), the in-cylinder NOx concentration was observed to be higher in the region between two sprays, relative to that within the spray core. This is attributable to a higher proportion of the aspirated gaseous fuel mixture being located between two sprays, resulting in higher gas temperatures from combustion, and hence elevated NOx formation rates. In the later stages of combustion there was much less distinction in the concentration of species between the two sampling locations

Investigating ethanol-gasoline spray characteristics using an interferometric drop sizing technique

To reduce reliance on fossil fuels there has been a global push to minimise fuel consumption, and incorporate the use of bio-derived fuels. In practical combustion systems that use liquid fuels, observing the spray behaviour of these biofuels is key in understanding fuel performance; in particular, droplet size distribution is known to have a strong influence on the fuel energy release and pollutant formation processes. This paper is aimed at the use of the TSI Global Sizing Velocimetry (GSV) interferometric technique as a method to gain detailed understanding of droplet number and size distribution, with a particular focus on ethanol-gasoline fuel blends. The imaging system for the GSV technique consisted of a Nd:YAG laser and a 4 MP (million pixel) camera, and the spray was generated using a generic automotive port fuel injector. The results showed that the GSV technique was able to effectively measure droplet concentration and diameters for all the fuel blends tested in this study. Ethanol was observed to have larger droplet diameters (both D10 and D32) as compared to fossil gasoline, with droplet diameters generally increasing as the proportion of ethanol in the gasoline was increased. The droplet concentration reduced with increasing radial distance from the spray centreline, but no appreciable change was observed with axial displacement from the nozzle tip. Degradation in the image quality was observed for fuel blends with less than 40% ethanol content. The GSV drop sizing measurements were validated using a mono-disperse droplet generator, and an excellent agreement (within 2%) was observed

An overview of the effects of fuel molecular structure on the combustion and emissions characteristics of compression ignition engines

Future fuels for compression ignition engines will be required both to reduce the anthropogenic carbon dioxide emissions from fossil sources and to contribute to the reductions in the exhaust levels of pollutants, such as nitrogen oxides and particulate matter. Via various processes of biological, chemical and physical conversion, feedstocks such as lignocellulosic biomass and photosynthetic micro-organisms will yield a wide variety of potential fuel molecules. Furthermore, modification of the production processes may allow the targeted manufacture of fuels of specific molecular structure. This paper therefore presents an overview of the effects of fuel molecular structure on the combustion and emissions characteristics of compression ignition engines, highlighting in particular the submolecular features common to a variety of potential fuels. An increase in the straight-chain length of the alkyl moiety reduces the duration of ignition delay, and the introduction of double bonds or branching to an alkyl moiety both increase ignition delay. The movement of a double bond towards the centre of an alkyl chain, or the addition of oxygen to a molecule, can both increase and decrease the duration of ignition delay dependent on the overall fuel structure. Nitrogen oxide emissions are primarily influenced by the duration of fuel ignition delay, but in the case of hydrogen and methane pilot-ignited premixed combustion arise only at flame temperatures sufficiently high for thermal production. An increase in aromatic ring number and physical properties such as the fuel boiling point increase particulate matter emissions at constant combustion phasing

Thermoacoustic Instability Considerations for High Hydrogen Combustion in Lean Premixed Gas Turbine Combustors: A Review

Hydrogen is receiving increasing attention as a versatile energy vector to help accelerate the transition to a decarbonised energy future. Gas turbines will continue to play a critical role in providing grid stability and resilience in future low-carbon power systems; however, it is recognised that this role is contingent upon achieving increased thermal efficiencies and the ability to operate on carbon-neutral fuels such as hydrogen. An important consideration in the development of gas turbine combustors capable of operating with pure hydrogen or hydrogen-enriched natural gas are the significant changes in thermoacoustic instability characteristics associated with burning these fuels. This article provides a review of the effects of burning hydrogen on combustion dynamics with focus on swirl-stabilised lean-premixed combustors. Experimental and numerical evidence suggests hydrogen can have either a stabilising or destabilising impact on the dynamic state of a combustor through its influence particularly on flame structure and flame position. Other operational considerations such as the effect of elevated pressure and piloting on combustion dynamics as well as recent developments in micromix burner technology for 100% hydrogen combustion have also been discussed. The insights provided in this review will aid the development of instability mitigation strategies for high hydrogen combustion

Polycyclic Aromatic Hydrocarbon and Soot Emissions in a Diesel Engine and from a Tube Reactor

An investigation into the exhaust emissions of carcinogenic polycyclic aromatic hydrocarbons (PAHs) from a diesel engine was reported. The study is reinforced by the experimental results obtained from a tube reactor aimed at examining the PAH formation processes from these fuels. The paper cantered on the 16 priority PAHs suggested by the United States Environmental Protection Agency (US-EPA). These PAHs were produced by burning conventional diesel fuel and a few binary fuels prepared by blending various proportions of toluene into heptane. Special consideration was given to the B2 subgroup of PAHs which are known human-carcinogens. Both the gas born (smaller) PAHs, as well as the larger PAHs, adsorbed onto the particulate were investigated. The engine used was a single-cylinder, light duty, high speed, diesel automotive research engine run at an Indicated Mean effective pressure (IMEP) of 7bar. Particulate matter was also produced in a tube reactor at temperatures ranging from 1050 to 1350 °C under pyrolysis (oxygen-free) conditions to study PAH and soot formation in conditions which resemble, to an extent, those found in the core of diesel engine fuel sprays. In the diesel engine, it was found that exhaust PAHs were influenced by combustion characteristics like heat release rates and ignition delay. However, in the quiescent oxygen-free conditions of the reactor, chemical composition of the fuels and temperature dominated PAH formatio